1. Field of the Invention
The present invention relates to an apparatus for controlling the steer angle of rear wheels of a vehicle of the type having steerable front and rear wheels (referred to as 4-wheel steering vehicle, hereinunder) and, more particularly, to a rear wheel steer angle control apparatus for 4-wheel steering vehicle, wherein an actuating mechanism for generating steer angle of rear wheels is controlled in accordance with the manner of operation of the steering wheel for steering the front wheels, thus automatically controlling the steer angle of the rear wheels.
2. Description of the Related Art
A known rear wheel steer angle controlling apparatus for 4-wheel steering vehicle, on which the present invention is based, is shown in Japanese Patent Laid-Open No. 44568/1982. This apparatus will be described hereinunder with reference to FIG. 2.
A shaft 2 is rotated in response to a rotational displacement of a steering wheel 1. The rotation of the shaft 2 is transmitted to a gear box 3 and is converted into a linear motion of linkages 4, which in turn causes knuckle arms 6 to pivot about pivot shafts 6a, so that front wheels 7 are steered at an angle .delta.f(t) as a function of time (t). The rotational displacement .delta.h(t) of the steering wheel 1 is detected by a sensor S1 mounted on the shaft 2, while another sensor S2 senses a lateral acceleration V which is generated in the vehicle in response to the rotational displacement .delta.h(t) of the steering wheel 1. A computer 8 activates an actuator 9 in accordance with the signals from the sensors S1 and S2, such as to impart linear motion to linkages 14 through another gear box 10. The linear motion of the linkages 14 causes knuckle arms 16 to pivot about pivot shafts 16a so that rear wheels 17 are steered at an angle .delta.r(t). The angle .delta.r(t) of steer of the rear wheels is determined by the computer 8 in proportion to the lateral acceleration V as follows. EQU .delta.r(t)=K.multidot.V (1)
Alternatively, the rear wheel steer angle .delta.r(t) is determined as the sum of the right-hand side of the above formula (1) and the product h..delta.f(t) of the front wheel steer angle .delta.f(t) and a proportional constant h as follows. EQU .delta.r(t)=h.multidot..delta.f(t)+K.multidot.V (2)
Apart from the known apparatus shown in FIG. 2, a 4-wheel steering vehicle has been proposed in which the steering device for steering the front wheels and the steering device for steering the rear wheels are mechanically connected to each other, wherein the angle of steer of rear wheels is controlled such that, when the rotational displacement of the steering wheel is small, the rear wheels are steered in the same direction as the front wheels, whereas, when the rotational displacement of the steering wheel is large, the rear wheels are steered in the direction counter to the direction of steer of the front wheels.
The known controlling apparatus for controlling the rear wheel steering angle in accordance with the lateral acceleration, however, does not sufficiently make use of the speed of operation of the steering wheel, so that this type of controlling apparatus does not provide any improvement in the response to the steering input for sharp turning of the vehicle at small radius of turning, although this controlling apparatus appreciably improves the running stability and facilitates the correction of steering operation during straight running of the vehicle at medium and high speed range by virture of the fact that the rear wheels are steered in the same direction as the front wheels regardless of the rotational displacement of the vehicle.
The other type of known controlling apparatus for controlling the rear wheel steer angle in accordance with the rotational displacement of the steering wheel involves a problem in that, since this type of apparatus also does not take into account the speed of operation of the steering wheel, the direction and angle of steer of the rear wheels is definitely determined by the rotational displacement of the steering wheel, regardless of whether the driver is expecting a sharp response of vehicle to quick steering input for averting from any obstacle or for lane change, or expecting a gentle turning of the vehicle as in the case of running along a curve of a large radius of curvature.
Thus, the conventional rear wheel steer angle controlling apparatus could not provide steering effect in accordance with the speed of operation of the steering wheel, thus failing to meet various steering demand of the driver.